Shock-resistant plastic compositions



United States atent 2,754,2s2 SHOCK-RESISTANT PLASTIC coMrcsrrIoNs William N. Stoops, Charleston, and Bernard A. Price, South Charleston, W. Va., assignors to Union Carbide and Carbon Corporation, a corporation of New York Application January 15, 1953, Serial No. 331,438 1 Claim. (Cl. 260-455) The physical properties of compositions based on thermoplastic resins are sharply dependent in general on the environmental temperature. Thus, such plastic compositions tend to become softer and more flexible as the temperature is increased and to become more rigid and brittle as the temperature is decreased. Thus, very few plastic compositions are capable of useful application over a wide range of temperatures. As a general rule, compositions which are adequately flexible and non-brittle at extremely low temperatures do not possess rigidity at room temperatures and above. Conversely, those compositions which are tough and rigid at room temperatures tend to be quite brittle at extremely low temperatures.

We have invented a plastic composition and a method of making it, which composition is tough and rigid at ordinary temperatures and which is free from brittleness at low temperatures. More specifically, our plastic composition has a brittle temperature below 20 C. and a stiffness modulus of elasticity at 30 C. of at least 50,000 p. s. i. In addition the plastic composition has unusually high impact strength and resistance to shattering, and these properties are retained at temperatures below 20 C.

The first step in the preparation of the plastic composition is to polymerize butadiene as essentially the sole monomer to a polybutadiene having a relativeiy high molecular weight. To achieve this, the polymerization of the butadiene is conducted in an aqueous emulsion in the presence of a polymerization catalyst and a temperature below 60 C. The next step in the preparation of the plastic composition is to polymerize styrene monomer in the presence of a specified amount of the polybutadiene. This second step may be termed consecutive polymerization and it is believed that a certain amount of the styrene becomes chemically combined as side chains on the polybutadiene base polymer. However, regardless of the validity of the theory, it has been established that the valuable combination of properties possessed by the new plastic composition is obtained only when the styrene is consecutively polymerized in the presence of the polybutadiene polymer; mere mechanical mixing of the polybutadiene with a styrene polymer giving products having high brittle points and inferior strength at low temperatures. Furthermore, it is essential to polymerize the styrene polymer in the presence of the previously formed polybutadiene; the reverse procedure in which the butadiene is polymerized in the presence of a previously formed styrene polymer giving inferior products.

One of the valuable characteristics of the present invention is that the valuable combination of properties of low. brittle point and high stiffness modulus at 30 C. is obtained over a wide range of polybutadiene content, thus permitting a selection of optimum properties. In the drawing Figure 1, curve A shows how the brittle point of the composition varies with the polybutadiene content; and curve A shows the corresponding variation in the stiifness modulus of elasticity. Thus, between a polybutadiene content of 23.5% and 38%, there may be obtained a wide range of compositions having physical properties varying from a brittle point of 20 C. and a stiflness modulus of about 180,000 p. s. i. to a brittle point of C. and a stiffness modulus of 50,000 p. s. i.

In order to compare the products of this invention with those of the prior art, curves B and B of Figure H show the variations in brittle point and stifiness modulus with polybutadiene content of a resin composition prepared by polymerizing styrene in the presence of various amounts of a polybutadiene rubber polymerized at C. under the conditions described in U. S. Patent No. 2,460,300. It will be noted that the slope of the curves B and B is extremely steep, indicating that minor variations in the polybutadiene content will have a pronounced eifect on both the brittle point and stifiness modulus. Thus, the properties of the composition are very sensitive to changes in the polybutadiene content, and it would be very diflicult to reproduce any given composition. Thus, the compositions would have a stiffness modulus above 50,000 p. s. i. and a brittle temperature below 20 C. over a very narrow range of polybutadiene content of 33.5% to 39%. Also, with the prior art compositions, it is impossible to produce compositions of very low brittle points around 70 C., and at the same time having a high stiifness modulus at 30 C. of around 100,000 p. s. i. Thus, the compositions of the present invention provide a much wider range of useful properties than do the products of the prior art.

Other prior art products are known in which styrene and acrylonitrile are copolymerized in the presence of rubber copolymers of butadiene and styrene. However these products as well as similar products in which styrene alone is polymerized on the same rubber, unlike those of the present invention, are brittle at low temperatures. Thus, the composition and properties of the polybutadiene component of the plastic composition are of great importance in obtaining the useful products of the present invention.

Thus, it is essential to use a polybutadiene which has been polymerized at a temperature below 60 C., for the reasons previously mentioned. Also, the lower the polymerization temperatures, the more suitable is the polybutadiene for making shock-resistant plastic compositions; polybutadiene rubber made at 10 C., for example, showing outstanding qualities. It is possible that the improved performance of this so-called cold-rubber is caused by a less highly branched structure. It is also important that the polybutadiene be of relatively high molecular Weight, as low molecular weight polybutadienes result in consecutive copolymers which are lacking in strength. Thus, the specific viscosity (an indication of molecular weight) of the polybutadiene should be at least 0.3 as measured with solutions of 0.2 gram of thepolymer in ml. benzene at 30 C.

(The specific viscosity is W 1) vise. solvent polybutadienes may be characterized by having Mooney:

plasticity numbers of 80 and above.

As noted in the drawing, the polybutadiene content-of the consecutive copolymer has a significant influence on the physical properties. Thus, in order to obtain compositions having brittle temperatures below 20 C. and a stitfness modulus of 30 Patented July 10, 1956 C. above 50,000 p. s. i. it is es-,

3 sential that the polybutadiene content of the consecutive copolymer be within the range of 23.5% to 38% by weight. The elTect of polymer composition on the physicalvpropertiesof the consecutive copolymer is shown-in the:following.ta ble.

this run is GRS standard'ruhher in composition For cmparison,=

68 parts. of styrene was polymerized.

andrpolymerization conditions. with 32 parts of this rubber.

BRITTLE TEMPERATURE, C.

The :brittle point of a plastic material is defined as the lowest permissible temperature at which a specimen may be i'bent at a specified rateof loading'withoutbreaking. A. S. T. M. .test D746-44T.'

T4 TEMPERATURE, C.

This: test determines the temperature at which the specimen :under examination shows a a stifiness modulus of 10,000 p. s. i. The procedures. used are amodification of the :A. S. .T. M. testDl943-49T Stifiness'Properties .of Nonrigid Plastics as a Function of Temperatureby Means, of a Torsional Test,-in which the testmachine weights and specimen thickness, specifications have been altered to permit measurement of stifiness modulus in the range of 10,000 ,p. s. i. rather than the A. S. T. M. prescribed range of 135,000 p. s. i; Actually the weights used are adjusted to provide one-half the torsionalforce specified by the A. S. T. M.-test and the specimen-thickness; specifications changedfirom, 40 mils to 70 mils. The T4 temperaturerdefines' approximately the upper temperature at which a semi-rigid plastic material retains appreciable strength.

STIFFNESS MODULUS AT 30 C.,-.P. S. I.

of using polybutadiene substantially free of copolymerized I styrene as a base for the consecutive copolymer. When a standard GRS-type rubber (copolymer of 75% butadieneand 25% styrene) was substituted for the pure poly-- butadiene, the-brittle temperature was +8 'C. instead of -60 to-78 C.

As previously indicated, the preparation of the'shockresistant. plastic compositions oi this invention involvesa fixed sequence of manufacture. The first step is the polymerization ofbutadiene to a polymer having theproperties previously. described. It is essential to conduct this polymerization at a temperature below 60 C., and in the form of an aqueous emulsion, but otherwise a wide variety-of polymerization conditions can be used, as are known in the art. The 'usual free-radical type polymerization catalysts are employed, such as organic-- peroxides, alkalipersulfates and diazoamino merization temperatures, the conventionalredox recipes are useful.

In-carrying out an-emulsion polymerization, for example, an-autoclave is purged of allzair by water and nitrogen displacement procedures and the -water,*emulsifier,=bufier; viscosity control agent and the reducing portion'of the oxidation-reduction-(redox)--catalyst, if such is used, are charged to the autoclave. Next, thebuta diene, modifiers and catalyst are added when the autoclave has beenbrought'to polymerization temperature, which benzene. -At low polymay be in the range from 40 C. to +60 C. The polymerization reaction is usually carried to about conversion, and the unreacted butadiene stripped off.

The second essential polymerization step is the polymerization of the styrene in the presence of the previously polymerized polybutadiene- The charge for this reaction consists of the polybutadiene latex, obtained as above, water, styrene, emulsifying agent and water. The usual procedure is to add the catalyst after the charge has reached polymerization temperature, which is usually 50 C., but may vary from 30 C. to C. The polymerization is carried out under nitrogen atmosphere and preferably carried to conversion. If not, the unreacted monomer may be removed by steam distillation. After the addition of any of the conventional rubber antioxidants, such as di-tertiary butyl para-cresol or sym. dibeta-naphthyl-para-phenylene diamine, the copolymerized rubberresin mixture is recovered by conventional precipitation procedures, using aqueous sodium chloride-sulfuric acid solutions, isopropanol-water-mixtures or acetic acidwater mixtures. To control the'rate of polymerization and provide, thermal control of the reaction, the styrene may be'added in increments.

The consecutive copolymers of this invention have im-;

pact strengths at room temperature (77 F.) of around 7 to 10 foot poundsper, inch of notch (Izod) and about 5 to 9 footpounds at temperatures .as low as .-20 F. The copolymers will not shatter whenstruck by 1a 10 pound steel ball dropped from a height of five feet,.;at temperatures as low as 40". F. 7

These properties maybe contrasted to thoseof the known. semi-rigid products resistance, which possessthis property at temperatures above 32? F.,-but are not shatter-resistant at lower tem-.

peratures. These known semi-rigid materials are prepared from mechanical mixtures of syntheticrubber with variousv resins, such as styrene-,butadiene rubber or acryloni--v trile-butadiene rubber mixed with polystyrene or styrene-- acrylonitrile copolymers;

It will be seen that the consecutive copolymer has equivalentstrength and toughness at room temperatureto commercial semi-rigid plastics, but is to be. distinguished therefrom by its flexibility and impact strength at low temperatures. This is unique in the field of plastics. For further comparison, plasticized vinyl filmis well known on the market for a variety of uses, yet evenwith specially prepared plasticizers, the brittle points of such film are in the range of 55 C..to 60 temperaturefiexibility in plasticized vinyl film, however, the degree of plasticization is such that theproduct is veryflexible and rubber-like .at room a flexural modulus of about 1000 p. s. i. Thus, it is seen that the present product is characterized by being tough and semi-rigid at room temperature, equivalent tocommercial products in its properties at room-temperature, and yet is as flexible at low temperatures as highlyplasticized wvinyl film. Stated in other words, therefore-the change in physical properties withchanges in temperatures of the new consecutive copolymers ismuch-less than the change which occurs with knownthermoplastic-com positions.-

Thev following. examples will further serve to charm-- terize the invention:

Example 1 A glass lined, autoclave maintainedat a temperature of 20 C. was charged with the following ingredients:

designed for high impact? C. To obtain such low;

temperature, having The polymerization reaction was stopped after 7.5 hours operation by which time 51% of the monomer had been converted to polymer. After stripping off unreacted butadiene, a benzene-insoluble elastomer was obtained.

A glass pressure bottle was charged with 93 grams of the above described rubber latex which contained 20 grams of polybutadiene. To this latex was added 40 grams of monomeric styrene, 38 grams of water and 0.5 grams of potassium persulfate. The pressure botfle was then turned end-over-end at 12 R. P. M. in a constant temperature bath maintained at 50 C. At the end of 17 hours, 98% of the styrene had been converted to polymer and the resin was recovered by conventional precipitation means after the addition of about 1.0% (by weight based upon the contained polybutadiene) of an antioxidant.

The vacuum dried consecutively copolymerized product, which contained 34% elastomer, showed a brittlepoint temperature of 64 C., T4 temperature of 88 C. and a 30 C. stiffness modulus of 139,000 p. s. i.

Example 2 A glass lined autoclave was charged with the following ingredients:

Parts Water 182 Butadiene 100 Technical sodium oleate (emulsifier) 5 Tertiary hexadecyl mercaptan (polymer viscosity The above formula is but one example of a coldrubber recipe and others known to the art are also useful. For example, other emulsifying agents, such as potassium oleate or the potassium salt of hydrogenated tallow acids may be used. Other polymer viscosity control agents include t-dodecyl mercaptan, primary dodecyl mercaptan or mixed tertiary higher alkyl mercaptans. Other freeradical type polymerization catalysts include organic and inorganic peroxides, such as diisopropyl benzene hydroperoxide.

The catalyst accelerators may be used either singly or combined. Tetraethylene pentamine, when used alone, yields a fast initial polymerization rate which thereafter decreases. Diethylene triamine, when used alone, yields a slow initial rate which thereafter increases. A combination of the two will yield an over-all constant rate. Also, periodically feeding fresh tetraethylene pentamine to the polymerization mixture will also yield an approximately constant rate.

The anti-foaming agent may be omitted until polymerization is complete and introduced prior to removal of unused monomer. Other anti-foaming agents may be substituted such as tetradecanol or other long chain alcohols.

In place of potassium chloride, other potassium salts or methanol can be used to prevent an undue rise in the viscosity of the autoclave charge. In place of potassium hydroxide, other pH control agents can be used to adjust the pH, such as sodium hydroxide or sodium or potassium carbonate.

A polymerization temperature of C. was maintained for hours at which time the reaction was terminated at a total conversion of 79%. The elastomer showed a specific viscosity at 30 C. of 0.998 as measured with a solution of 0.1 gram of elastomer dissolved in 50 ml. benzene.

To 30 parts of this elastomer in latex form were added 70 parts of monomeric styrene, 0.5 part potassium per- 5 sulfate as catalyst, 0.5 part of technical sodium oleate and sufiicient water to make a total of 300 parts. A polymerization temperature of 50 C. was maintained for 2 hours at which time all the monomeric styrene had been converted into polymer. A small quantity of antioxidant was added to the latex and the resin was then recovered using conventional procedures. The composition contained 30% polybutadiene.

The recovered product showed a brittle-point temperature of -60 C., a T4 temperature of 94 C., 30 C.

15 stilfness modulus of 116,000 p. s. i. and Izod impact values as follows:

. Impact Test Temperature, F Strength, Ft. Lbs. Per Inch of Notch Example 3 A glass-lined autoclave of -gallons capacity was charged with the same proportion of ingredients detailed for the butadiene polymerization in Example 2. The polymerization temperature was varied between 10 C. and 13 C. for a total of 14 hours at which time 75% of the butadiene had been converted to polymer which showed a specific viscosity at C. of 0.78.

To parts of this elastomer in latex form were added 7 65 parts of monomeric styrene, 0.5 part of potassium persulfate, 0.1 part tertiary hexadecyl mercaptan and 0.5 part of technical sodium oleate. A polymerization temperature of 50 C. was maintained for 3 hours at which time the reaction was stopped with 97.5% of the monomeric styrene having been converted to polymer. Antioxidant was added to the latex and the resin was recovered using conventional procedures. The product contained 35.6% of elastomer and showed a brittle-point temperature of -78 C., a T4 temperature of 78 C., a 30 C. stiffness modulus of 72,000 p. s. i. and impact values as follows:

Impact Strength, Foot Pounds Per Inch of Notch Test Temperature, F.

Example 4 ""30 CTstiffness 'niodulus ofl66g000 p. s. i." and impact values as follows:

. Impact Foot Pounds 1 l er-Inch of Notch A' glass-lined autoclave maintained atI10. C. was' charged with the following ingredients:

*Parts Water 182 '.'-Butadi'ene 100 Tertiary hexadecyl mercaptan 0.75" Technicalsodiumoleate 0.5 JPotassium hydroxide 0.75 '5 :2-ethylhexanol 0.75 & JBenzene 2.4

The catalyst consisting of cumene hydroperoxide (0.75 part), tetraethylenepentamine (0.29 part), and diethylehetria'm'ine' (0129 part)-';-" was "divided" into-four equal parts." One portion of theca talyst wa's "charged to'the autoclaveinitially and the'-"-remainingportions 'were,

charged at 5 hours intervals thereafter. At the end of 20 hours operation 76%o'f'themonomer had been converted into polymer. .The polymer was 58% soluble in -benzene.

TilTo'l30-p'arts 'of this polybutadiene ih latex. form was,

. added 70 parts of monomeric'j'styrene, sufiicient water -todmake' '400-part's'; '05 part of technical sodium oleate, 0.5 part of potassium persulfate and 0.1 part of tertiary hexadecylr rnercaptana A. polymeriiation temperature" of 50 C. was maintained-for tia'lly100%of thestyrene hadlbeen converted into poly- -.mer.""The recovered product, containing"3 of. the 'elastomer', showed a brittle-point temperature of 30' Q, T temperature'of89" C., 330 C. -stiflness modulus of 128.000 p. s. i and Iz'od impact values as 'follows:

Impact Strength, Foot -Pounds Per Inch of Notch Test Temperature; F

Examples 6 "and 7 *In-cont'rast to the examples given above which illustr'at'e the methods used in obtaining the products of our invention which showexcellent low-temperature proper-.

:. ties, these two examples describe the polymeriz'ation of E-the prion-art"products described'in' U: S. 'Patent No.

2,460,300; and their physical properties.

' Strength,

s pa s r OBCDNQQ '5 hoursby which time essen- 7 A glass-lined autoclave-was charged with the following ingredients:

Y Parts Water 530 1' Nopo '1216 (sulfated oil emulsifier) a a 5.3

'.Hydrogen peroxide 1.2

Ferric nitrate 0.007

Nitric acid 0.35

: Physical propert-y 'data -are present in the table below.

Izod Impact, Ft. Lbs/Inch Brittle- T 30 0. Notch Exampoint .Temp Stiff-.. a. a. we

ple Temp., 0. Mod., H V

' C. p.'s. i." 77 13. 50F. I 0 F; =-I2 0 From a comparison of the product ofiExample 4which 35 hasastifinessmodulus of166,000 p. s. i., with the prior art product of Example 7, which also shows a stiffness modulus of 165,000 p. s. i. it is apparent'thaton a-basis oft, equal stifiness, the material of our invention is superior to the prior art product in impact strength and toughness 4o (lack of brittleness) over a wide range of temperatures.

! What is claimed is: I y

Process .for makingv synthetic resins which comprises emulsifying butadieue as essentially the sole monomeric l materiab in-water' containing an emulsifyingagent and a free-radical. .type polymerization catalyst, polymerizing -the butadiene at a temperature below about 60 Cato form apolymer having a specific viscosity above about 0.3, adding monomeric. styrene to the polymericlatex thus formed," and polymerizing the .styrene the presence of thebutadiene polymer to form a synthetic resinh aving as its ultimate composition f rom-2a5f7q to 38% by weight of butadiene, and. from 62% to-76.5%; by weight of Lstyrene; sald synthetic riesi nhaving a brittle temperature below '20 C.-and a stiffness modulus at 30 C, of at least 50.000 p. s. i.

Referencs'Citd in the'file of this patent 'STAT'ES PATENTS 2,460,300 Le Fevre et a1. Feb 11 1949 

